Blind via laser drilling system

ABSTRACT

A laser drilling system for producing blind vias ( 13 ) in multilayered circuit boards ( 9 ) includes an RF excited sealed carbon dioxide laser oscillator ( 19 ), X-Y motion table for moving the boards during on-the-fly via formation, and a controller. A laser beam ( 20 ) is focused beneath the circuit board by a long focal length lens ( 17 ) so as to be defocused at the surface of the board. A slotted mask may be used to allow drilling of multiple vias simultaneously, air may be injected to the via to assist in material removal, glass scales may be used for feedback position control, both the mask and table may be rotatable, and a computer may be used for determining an optimal processing path.

This application claims benefit to Provisional application Ser. No.60/019,140, filed Jun. 5, 1996.

BACKGROUND OF THE INVENTION

The present invention relates to a laser system and method of formingeconomical and reliable blind vias in circuit boards and polymer basedmultichip modules.

Laser drilled blind vias are constructed by positioning and pulsinglaser beam radiation over a pre-etched window to remove dielectricmaterial. The use of pre-etched windows as a mask for laser drillingmultilayer circuit boards is disclosed in U.S. Pat. No. 4,642,160.

The construction of multilayer circuit boards and the processes used toproduce them are well understood. Through vias or holes thatinterconnect one side of a circuit board completely to the other andthat have been made conductive have been the Z-Axis interconnecttechnology choice for multilayer circuit boards for years. These holesare typically mechanically drilled in stacks on numerically controlledmulti-spindle drill machine. The common practice of using leadedcomponents allowed interconnections to be made in the through holes andthe need for blind vias was reduced. Surface mount technology where thecomponent leads make interconnections on the surface instead of in theholes actually increases the demand for through vias for electricalinterconnections to internal layers in multilayer circuits. As surfacemount components increase in pin or lead counts, the pin density becomecloser. The dense component placement and dense pin count on multilayercircuit boards and polymer based multichip modules creates aninterconnect density problem in the Z-Axis. This problem is called viastarvation.

One solution for this via starvation is blind via technology as depictedin FIG. 1 and FIG. 2 as via 13 interconnecting down one layer and via13′ interconnecting down to layers two and three. The demand for smallerdiameter Z-Axis interconnections coupled with the increased number ofinterconnections has made the mechanical drill process the most costlysequential process step in the manufacture of multilayer circuit boards.

Integrated circuit technology and component packaging have created atechnology demand on the circuit board and polymer multichip moduledesign world. Fine pitch quad flat packs (QFP) (FIG. 15) with 0.305 mmcenters and ball grid arrays (BGA) (FIG. 13) lead the packagingchallenge as the component footprints are designed with finer I/Opitches as shown by microBGA's in FIG. 14. The preferred solution forinterconnecting these dense component footprints on the circuit boarddemands blind via technology.

Blind via technology has been available for many years but the complexprocesses needed to produce these blind vias have typically doubled thecost of a circuit board or polymer based multichip module. Fourdifferent blind via technologies are known and practiced, but all ofthese interconnect improvements increase the overall costs of thecircuit board and have not openly received broad industry acceptance.

The best known of the four blind via technologies is mechanical drillingwhich creates the need for sequential layer lamination. Mechanicaldrilling has been the primary barrier to the growth of blind viatechnology in the circuit board industry. The economic constraints aredue to the lengthy time required to drill small diameter vias and thedifficulty to control Z-Axis depths FIG. 11. It is well known that thesmall holes, less than 0.254 mm diameter, produced by advanced precisioncomputer numerically controlled (CNC) mechanical drills, are drilled atonly one panel high. Furthermore, the advanced CNC drill producesapproximately 1.5 hits per second per drill spindle. Therefore, a fourspindle advanced CNC drill equates to about six hits or vias per second.FIG. 9 and FIG. 10 compares the sequentially laminated mechanicallydrilled method to the laser drilled method used by the laser systeminvented in this disclosure. The general processing of the panels forlaser drilling in this invention uses the conventional process steps ofFIG. 9. Different size vias require different size drill bits andprocessing time is required to change these drill bits. Anothertime-consuming characteristic of the current state of the art mechanicaldrilling is the requirement for the x/y table to come to a complete stopin order to eliminate very small drill bit breakage. This zero movementrequirement increases the cost of the CNC drill equipment and is theprimary contributor to the average 1.5 hits per second per drill spindleoutput.

Two batch process blind via technologies are plasma etching and photovia. Plasma etching uses copper as a mask and photovia typically doesnot. Both plasma etching of blind vias and photovia process are limitedto interconnecting one layer down due to the chemical mechanism used forremoving dielectric and are called batch processes. Multiple depthprocessing is accomplished through sequential layer processing, withmultiple layer interconnections achieved by Z-Axis daisy chains. Thisprocess for making multiple depth blind via interconnections is calledsequential buildup technology. Plasma etching demands a dielectricpolymer that can readily be chemical etched. This creates somelimitations in material selection, similar to the laser processingdescribed in this disclosure, but to a greater extent. Close processcontrol is necessary in order to not overetch the via crater and createa barreled via that is considerably more difficult to metallize.

Even more material limitations are imposed by photovia blind viaprocessing. Photovia materials require a dielectric polymer that hasbeen chemically processed to be photo-sensitive to generally visiblelight, a ultraviolet light or near ultraviolet light. Thesephoto-sensitive polymers tend to be quite expensive and generally do notmeet the requirements for higher performance circuits. Most photoviaprocessing requires additive or semi-additive plating which yields lesssurface adhesion. This reduced surface adhesion is considered dangerousfor most surface mounted components, especially where ball grid array,flip chip and chip scale packaging technology is the chosen componentinterconnect scheme.

The fourth blind via technology is laser based. Several different lasertechnologies including Eximer, Nd:YAG (neodymium-doped,yttrium-aluminum-garnet) and CO2 (Carbon Dioxide) have successfullydrilled blind vias in circuit boards. The main limitation has been thequantity of panels that can be processed by a single laser system. Thisis best understood by calculating the blind vias drilled per minute orper second on a given panel. Each panel may have one or many replicatedcircuit designs that will yield finished circuit boards. Both the Eximerand the Nd:YAG, have extremely small beam sizes—smaller than thediameter of small blind vias. Eximer lasers are quite expensive to runand require ongoing maintenance. Making blind vias of 0.102 mm to 0.203mm diameter require a complicated trepanning procedure with the Nd:YAGlaser. The beam is moved in a spiraling fashion until it removes orablates the dielectric material from the entire diameter of the via.Another limiting factor with a Nd:YAG beam is that the beam energy isreadily absorbed by the copper clad material and, therefore, has to beprocessed at an extremely low energy level and consequently pulsedmultiple times while trepanning. The Nd:YAG laser requires rechargingafter 60 hours of use.

The sealed RF excited CO2 laser system described in this disclosure hasa 20,000 hour life before recharging according to the supplier Synrad,Inc. Another type of CO2 laser system, uses a Transverse ExcitedAtmospheric (TEA) mechanism to control the pulsing of the CO2 laser. TheTEA CO2 laser system produced by Lumonics that is designed for laserdrilling blind vias is limited to 150 pulses per second plus with thelow 40 watt laser, it is only capable of vaporizing 0.0254 mm ofdielectric material with a single pulse. In other words, 0.127 mm ofdielectric material would take 5 pulses and the pulse limitation wouldbe at 30 pulses per second.

Early applications of CO2 laser drilling have typically been difficultwhere the composite materials have vaporization states that aredissimilar. One such example is the common circuit board material ofepoxy and woven glass fibers commonly referred to as FR4. In order tovaporize these dissimilar materials, a focused laser is used where theenergy needed to effectively remove the material in the via is governedby the success of removing the higher state of material. In order tokeep from damaging the mask or blind pin (bottom of the via), thefocused laser system must be pulsed several times over a single opening.This multiple pulse condition generally governs and extends the amountof time the beam has to be positioned over the masked window. In mostcases, the time to produce multiple pulses demands the motion system bestopped and stabilized to effectively vaporize the laser drilled blindvia. To further complicate matters, laser processing of circuit boardsgrew out of the ceramic hybrid circuit industry where high power focusedlaser systems with short focal length are the norm.

All of the companies that are producing blind via circuit board laserdrilling systems, including Electro Scientific Industries (ESI),Lumonics, Convergent Energy, Mitsubishi, Hitachi and Panasonic, use aGalvanometer or modified Galvanometer for beam positioning and all use arelatively small beam size. The Galvanometer that is used to rapidlyposition the beam in a small area (from: 0.762 mm×0.762 mm up to 50.8mm×50.8 mm), demands that the X/Y positioning table come to a completestop before using the Galvanometer for local drilling. This addsadditional time for stabilizing the Galvanometer, and can add up to 200seconds to the drilling cycle. In many cases, the drilling time of thelaser system embodied in this patent disclosure will be less than thetime to move and settle the table with the Galvanometer. The most commonalignment for theta is to make an alignment adjustment in software. Thistheta software adjustment forces the table to continually use two axisbefore coming to a complete stop. The Galvanometer technology makesincorporating the debris removal technique shown in FIG. 6 and FIG. 20of this disclosure, difficult if not impossible.

Polymer dielectric materials with improved thermal and electricalcharacteristics are now available and several are produced withmaterials that can be categorized as having comparable vaporizationstates. Examples are epoxy and polyimide resin impregnated non-wovenaramid dielectric materials and liquid crystal polymers.

The aramid material produced by DuPont Fibers is called Thermount® andis the preferred dielectric material for the system and method disclosedin this invention. Multiple material laminators or treaters areprocessing versions of epoxy and polyamide coated Thermount®. These U.S.suppliers include NelTec, Arlon, and Polyclad. In addition, Thermount®provides improved thermal stability for chip on board, flip chip, andchip scale packaging.

Another material that was developed in conjunction with an earlierpatent (U.S. Pat. No. 4,642,160) is currently commercially availablethrough AlliedSignal Laminate Systems. This material called RCC® is atwo part epoxy coated copper foil. The epoxy next to the foil is in theC stage or cured and the other epoxy coating is in the B stage orprepreg as noted by the circuit board industry.

A Bismaleimide Triazine (BT) resin coated material that uses a yet to bedisclosed non-woven organic fiber similar to the non-woven aramid, iscoming out. One such circuit board material is produced by MitsubishiGas Chemical Co., LTD (MGC) and is called FoldMax™. This material isalso economically laser drilled with the laser system embodied in thisinvention.

The current limitation in laser drilled diameter is greatly affected bythe ability to cost-effectively deposit metal or plating into the blindvia's low current density area. The limitation appears to be at the 1:1window diameter to dielectric depth, especially below 0.076 mm diameterand depths. Several known methodologies can readily plate into thesmaller cavities readily produced by laser drilling such as ultrasonicplating, reverse current and laser plating. The most common solution isto use the electroless plating methodology, eliminating low currentdensity conditions with solution movement. This electroless platingmethod, while capable, does typically add process time and when iteliminates electroplating, it is not practical for large volume panelproduction. Direct plating, where the polymer surface is made conductiveby carbonization, is an acceptable metal conducting method whereelectroplating can be used to cut down the metallization time.

The laser system and method for rapidly and reliably drilling blind viasat multiple depths described in this invention allows blind vias to beeconomically introduced into interconnect packaging designs.

SUMMARY OF THE INVENTION

It is the object of this invention to provide a cost-effective laserdrilling system for drilling blind vias in multilayer circuit boardpanels and polymer based multichip module panels at such a rate thatwill make the laser drilling method for multi-depth blind via drillingeconomically feasible. The system is designed to create the same orbetter output in panels per hour than a multi-spindled mechanicaldrilling machine.

One aspect of the invention is a laser drilling system comprising an RFexcited CO2 laser of at least 100 watts, preferably 200 watts or more,with the capability to pulse an infrared beam at least 5,000 andpreferably 10,000 times per second at zero duty cycle. An X and Y axisposition control system is used which includes a table for supporting apanel relative to the laser for drilling and a motion controller formoving the panel under the laser. A controller, preferably underdirection of a computer or workstation which programs traversal, outputsa signal to the motion controller and receives location input from thecontroller to actuate the laser. Preferably, glass scales are used onthe table that detect position in both X and Y axes and feed back to thecontroller position information to pulse the laser as it crosses orenters the etched windows. The system preferably includes optics forfocusing the laser beam below the panel.

Another aspect of the invention is a blind via laser drilling method fordrilling blind vias in a printed circuit board or multi-chip modulesubstrate, comprising laser drilling through a pre-defined mask thatdefines the size of the blind vias on circuit boards and polymer basedmultichip modules, using the reflective characteristics of the mask overthe dielectric material and a blind pin under a layer of the dielectricmaterial by IR laser vaporizing dielectric material in the substrate ina single pulse per via. The laser beam is produced by pulsing a CO2laser which has a power of at least 100 watts and the beam is focusedthrough the substrate to a depth sufficient to broaden the laser beamdensity at the substrate surface, allowing the radiant beam to reflectoff a copper mask on the surface of the panel and also reflect off ablind pin at the bottom of the blind via.

The invention enables the production of printed circuit boards and thelike with surface mount pads, such as a ball grid array, havinginterconnections down to multiple layers, each multilayerinterconnection being contained within one of pads. The invention alsoenables the manufacture of circuit boards with interlayer, multilayer,and terminating resistors, formed in the laser-drilled vias.

The foregoing and other objects, features and advantages of theinvention will become more readily apparent from the following detaileddescription of a preferred embodiment of the invention which proceedswith reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a circuit board panel with two blindvias after laser drilling and a third blind via not yet laser drilled.

FIG. 2 is a cross-sectional view of a circuit board panel with blindvias for interconnections between layers 1 to 3; between layers 1, 2 and3; and between layers 1 to 2.

FIG. 3 is a schematic drawing of a preferred embodiment of the blind vialaser drilling system of this invention, including lens configuration,motion control and laser firing control by glass scale positioning.

FIG. 4 is a schematic drawing of the lens positioning and focus point ofthe beam delivery system of FIG. 3 required for rapid laser beam pulsedelivery.

FIG. 5 is a cross-sectional view showing the watt density distributionthat enters the etched window allowing the polymer dielectric materialto be vaporized, forming a plateable blind via, in the method using theblind via laser drilling system of FIG. 3.

FIG. 6 is a cross-sectional view of the flow of air and vacuum thatenters the window during vaporizing which minimizes debris and charringin the method of this invention.

FIG. 7 is a cross-sectional view similar to FIG. 5 showing the oversizelaser beam and reflected laser energy at both the base of the blind viaand at the surface of the masked window.

FIG. 8 is a cross-sectional view of a resistive polymer being screenedinto a laser drilled blind via according to the method of thisinvention.

FIG. 9 is a flow diagram of the conventional sequential laminationprocess for producing mechanically drilled blind vias.

FIG. 10 is a flow diagram of the laser process according to the presentinvention and used with the laser system of FIG. 3.

FIG. 11 is a cross sectional view of a mechanical drill that is drilledpartly into the panel for controlled Z-Axis drilling.

FIG. 12 is a cross-sectional view of a conventional sequentiallamination process for producing mechanically drilled blind vias.

FIG. 13 is a ball grid array footprint on the surface of a circuit boardpanel with a laser drilled blind via in the center of each pad accordingto the invention.

FIG. 14 is a is a micro-ball grid array footprint on the surface of acircuit board panel with very small laser drilled blind vias in thecenter of each pad according to the invention.

FIG. 15 is a quad flat pack footprint on the surface of a circuit boardpanel with laser drilled blind vias in the center of each pad accordingto the invention.

FIG. 16 (top) is a plan view showing a Ball Grid Array and fine pitchQuad Flat Pack and traversal plan depicting the serpentine flow from thesoftware optimization program for laser drilling the vias according tothe invention.

FIG. 17 is an improved traversal plan after the use of softwarescripting to further optimize the travel path in the softwareoptimization program.

FIG. 18 is a perspective view of an off-contact slotted mask allowing aportion of the laser beam to be selectively directed to the etchedwindows according to the invention.

FIG. 19 is a drawing of the top of an off-contact slotted mask similarto that of FIG. 18 over an exhaust chamber for debris removal.

FIG. 20 is a drawing of a rectangular air nozzle system attached to thebottom view of the off-contact slotted mask.

FIG. 21 is a cross sectional view of the off-contact slotted mask withexhaust chamber.

FIG. 22 is a perspective view of a circuit board panel with an array ofetched windows and two large etched windows at each end of the panel fortheta alignment of the circuit board panel according to the invention.The dielectric material has not been laser drilled from within the twoalignment windows.

FIG. 23 is a cross sectional view of the panel of FIG. 2 with analignment system according to the invention showing schematically twoCCD cameras located at two preset tooling positions on either side ofthe laser beam delivery tube with the off-contact slotted mask andexhaust chamber on the bottom of the laser beam delivery tube. Thedielectric material within the two alignment windows have not been laserdrilled.

FIG. 24 is a cross sectional view like FIG. 23 showing the laser beamdelivery tube with the off-contact slotted mask and exhaust chamber onthe bottom of the laser beam delivery tube aligned over one of thealignment windows. The dielectric material within the two alignmentwindows has been laser drilled.

FIG. 25 is a perspective view similar to FIG. 23, but after thedielectric material has been laser drilled from within the two alignmentwindows to expose a buried pin on one of the inner layers.

DETAILED DESCRIPTION

Overview of System and Process

In accordance with a preferred embodiment of the present invention, alaser system is constructed by integrating a sufficiently powerful(preferably ≧200 watts, more preferably 500 watts) sealed carbon dioxide(CO2) laser emitting an infrared (10.6 micrometer) laser beam pulsed byusing radio frequency controls (RF excited) capable of pulse ratesgreater than 10,000 per second at a zero duty cycle. Synrad, Inc. ofMukilteo, Wash., U.S.A., and Coherent, Inc. of Santa Clara, Calif.,U.S.A., make lasers that can be adapted as described herein. RFcontrolled CO2 lasers have been used in the medical profession becauseof the ability to rapidly pulse the laser beam and not appreciably harmtissue. This characteristic can be transferred to laser drilling ofpolymers under controlled conditions as described in this invention.While the beam delivery rate of the RF controlled CO2 laser typicallyleaves less residue than the Continuous Wave (CW) CO2 laser systems, itdoes leave excess debris, fibers and charred polymer unless a highpressure stream of clean air is pointed into the blind via and extractedby a vacuum system. It was originally thought that inert gases such asArgon or Nitrogen were necessary to eliminate charring. Applicant'sexperiments have eliminated via wall charring by using clean highpressure air focused into the etched relief in the mask. The mechanismis believed to be similar to the effect of the air or oxygen blast in aacetylene cutting torch. Without this high stream of air the taperedwalls are not concentric but oval.

RF controlled CO2 lasers used in medical surgery are typically less than50 watts and not powerful enough to remove dielectric material.Furthermore, the laser beam in medical applications delivers multiplepulses in a focused fashion to remove tissue at the focal point makingit economically impractical to rapidly remove dielectric materials inblind via circuit board applications. Even though sealed RF controlledCO2 lasers are available for industrial applications in the 200 to 700watt range, a new laser system and processing method is needed to makedrilling blind vias economically feasible.

This invention involves a moving laser beam delivery system that createsa watt density sufficiently strong enough, yet wide enough, to vaporizedielectric material through a window in a mask down to different depthblind pins 13 and 13′ with one pulse per via in a single pass. While themechanism for removing dielectric material can be measured as Joules ofenergy, this invention allows for the delivery of a consistent energylevel across the window opening, plus the energy level remainsconsistent down into the via. The key to doing this is to focus the beamwell below the intended maximum depth of the via, preferably below theprinted circuit thickness. The watt density of the beam into thedielectric material needs to be sufficiently strong enough to vaporizethe polymer dielectric material but low enough not to harm the copper orother window mask surface 12 or the blind pin 16 and 16′. This inventionallows multiple size etched windows or blind via diameters along withmultiple depth blind vias to be produced with the same energy levelwithout re-focus or pulse duration adjustment.

Laser control technology presently exists to rapidly change both thefocus and pulse duration which will not adversely effect the viaformation, but, it is not clear where the limitation of vias per secondwill be with focus and power adjusts on the fly. Regardless, the presentinvention avoids these limitations. Effective laser drilled blind viascan be made on the fly down to levels four, five and six, using thepresent invention. The current limitation is not the effectiveness ofthe laser energy source to vaporize polymer dielectric, but theinability to reliably electroplate high aspect ratio laser drilled blindvias.

An early form of laser drilling of blind vias is described inapplicant's prior U.S. Pat. No. 4,642,160. The described method andsystems then available were insertable for laser drilling at multipledepths, drilling on the fly using a single pulse per via. This inventioncarries forward additional process improvements primarily through theinvention of a laser drilling system allowing the method described tocost effectively be fabricated on a production basis.

FIG. 4 depicts a long focal length lens 17 ranging from 127 mm to 254 mmwhen focused between 2.54 mm to about 50 mm below the top surface of thecircuit board etched window 22, changes the Gaussian curve for the beamcreating a broad watt density curve 23 as shown in FIG. 5. The amount ofenergy that is allowed to enter the etched window is controlled by pulseduration. Pulse duration rates between 800 microseconds and 2,500microseconds depend on the volume of dielectric material to be vaporizedwith a 200 watt sealed RF controlled CO2 laser with the ability to pulseat 10,000 per second (at zero duty cycle) can deliver between 1,250 viasper second and 400 vias per second. A 700 watt sealed RF controlled CO2laser with the ability to pulse at 10,000 per second (at zero dutycycle) can deliver up to 3,000 vias per second.

The invention further embodies a useable beam at window entry 13 and 13′that can be as large as three times the size of the etched window. Thissize will allow an effective 0.76 mm diameter wide beam to move rapidlyand continuously over a 0.25 mm diameter etched window. The proper wattdensity is controlled by the de-focus of the beam into the panel 22 andaided by the long focal length lens 17. FIG. 7 depicts the mechanism ofbeam overlap of etched window used within the invention fordrilling-on-the-fly. The limitation on how small a diameter of the blindvia can be produced by this invention is governed by the ability to etchthe appropriate window 13 and 13′ and its use as an interconnect isfurther limited by the processes that are used to make the blind viasconductive, such as electroless or direct plate and subsequentelectroplating. These limitations are known and outside the scope ofthis invention. The tapered blind via wall 14 is essential for therelease of gasses that can be trapped in blind vias during the multiplesteps in the metallization process.

To provide a totally reliable clean blind via for plating, a vapor honeset at an appropriate pressure so as not damage the copper mask used inlaser drilling can be used to clean any fibers and condition the blindvia for metallization. This vapor hone uses a slurry of 400 gritaluminum oxide under high air and water pressure to not only to cleanthe laser drilled blind vias, but also cleans any debris physicallydeposited on the surface around the windows 13 after laser drilling. Twomanufacturers of vapor hones are Vapor Blast Mfg. Company and PressureBlast Manufacturing Company. The vapor blast process step followed by amicro-etch to clean the copper on the blind pins 16 and 16′ allows forAutomatic Optical Inspection (AOI) of the copper pad 16 and 16′ whichwill assume reliable plating adhesion into the blind via. AOI is alsosuggested as a quality measuring tool to assure the windows 12 have beenproperly etched prior to laser drilling 13 and 13′.

Surface mount technology has brought the ability to place components ina much denser fashion than the previous leaded through hole packaging.The capability for denser component placement is challenged by thedemand for more control of transmission lines and the need for moreterminating resistors. These terminating resistors used in surface mounttechnology are typically cap resistors and are surface mount soldered.While the resistors are not generally expensive, the volume of resistorsneeded and the effort needed to place and solder them on the panelcreate increased costs but most importantly take up valuable space onthe surface of the circuit board. An outgrowth of the economicallydrilled blind vias embodied in this invention is the ability to screen29 a resistive polymer 28 and 30 into a blind via that has been laserdrilled to a diameter predetermined by the window size and a depth thatcan extend through multiple layers. Such a blind via typicallyterminates at an internal ground plane as shown in FIG. 8. Since theseterminating blind vias 28 do not have to be plated, the window openingsdo not have any minimal size requirement. Therefore, via diameter andlength as well as material resistivity can be used to controlterminating resistance values. Terminating resistive polymer materialsthat are used in the ceramic hybrid circuit industry are available forthe laser drilled terminating resistor.

The cost effective laser drilled blind vias produced by this inventionmay be used to produce terminating resistors on the surface of the panelas shown in FIG. 8. Windows etched for this feature are calculated byunderstanding the volume of resistive material, but primary resistancecontrol is within the mixture of the polymer resist 30. Two methods maybe used to deposit the resistive material into the laser drilled blindvias. One method is to flood the surface with resistive material andsqueegee 29 this material into the laser drilled blind vias by dragginga hard sharp squeegee over the entire surface. A second method would touse a screen with relief openings over the laser drilled vias 31 thatwould allow the resistive polymer to selectively enter the vias definedas terminating resistors. The second method would allow all the blindvias to be laser drilled at the same time. The blind vias selected to beconductive would be masked by the screen as the terminating resistorvias are filled with polymer material.

A off-contact slotted mask 39 as shown in FIG. 18 allows a slottedsection of the beam 20 to pass through the slot 40. The slotted mask ismade of a material that will absorb that portion of the laser beam thatdoes not pass through the slot. The resulting elongated narrow beam cannow be rapidly move in the direction of its longest opening allowing thelaser beam to be pulsed over an row of etched windows. This mask allowsthe defocused beam used in the present invention to travel in one axiswithout hitting a nearby row of etched windows. The panel is moved underthe slotted mask 39 in one axis, then the slotted mask, or the X/Ytable, is rotated 90 degrees and the table with the panel is moved inthe opposite axis. The off-contact slotted mask is made of asufficiently thick metal, preferably Aluminum, to absorb the defocusedbeam without creating an absorbed temperature to distort the mask.Refrigeration cooling may be used to cooling the off-contact slottedmask by running the heat exchanged coolant from the RF Power Suppliesand RF Excited CO2 laser. The pre-optimized travel path is set to movethe table in one axis at a time with the laser beam pulsed at theappropriate pulse duration (e.g., 0.0005 sec. to 0.003 sec for nonwovenaramid dielectric of thickness ranging from 0.05 mm to 0.2 mm). Thetable speed for a given pulse duration is calculated for the diameter ofthe etched window 12 and the dielectric thickness 11 (e.g., 760 mm/secto 250 mm/sec for a 200 watt laser).

A disbursement of clean dry air as in FIG. 6 is delivered in the slottedmask 39 of FIG. 20 to the rectangular via cleaning attachment 43 so thata force of clean dry air is jettisoned into the etched windows 12 whilethe laser beam is pulsed. The air acts like the heavy gas flow in acutting torch and forces debris and other materials from the laserdrilled etched windows 13 and 13′, resulting in a laser drilled blindvia that will not need to be post cleaned or will need very little postcleaning. Just below the slotted mask 39 and the air jet rectangular viacleaning attachment 43 is a small chamber used for exhaust and debrisremoval.

Registration and theta alignment can be accomplished as shown in FIGS.23 and 24 by moving the panel under to preset CCD Alignment cameras 49and 49′ allowing a rotating theta table to move the panel over or underthe X/Y table. The two large etched windows 45 at each end of the panel,shown in FIG. 22, can be used to average the registration throughcurrently available software and vision hardware. Once the panel thetais set, the table is moved under the laser beam delivery tube 48 asshown in FIG. 24, and the laser is pulsed with a long enough pulseduration to remove the dielectric material as shown in FIG. 25, exposinga buried pin 47. This buried pin can be the same etched spot that isused by many circuit board fabricators for inner layer alignment in theMultiLine Four Slot System. Other commonly used fiducial targets may beused as buried pins for alignment. Once the dielectric materials havebeen removed from both tooling etched windows 46, the table may move thepanels back under the CCD Cameras (or a camera with beam-splittermirrors) as shown in FIG. 23 (without dielectric material removed) and aquality check can occur. If the buried pin 47 is located within atolerable range within the laser drilled etched window 46, the panel canbe aligned to the first etched blind via window for the circuit designand laser drilling can commence. If the buried pin 47 is outside atolerable range within the laser drilled tooling etched windows 46 thepanel may be rejected and removed from the system. Theta alignment usingtwo pins 47 thus provides an optical alignment technique that enablesthe laser drilling process to be moved rapidly along a traverse in asingle axis.

Description of Apparatus

Referring to FIG. 3, the focal length of the focusing lens 17 is shownas an unknown distance 21 from the top of work piece 9. This focallength of the focusing lens is critical to allow an appropriate wattdensity distribution 23 as shown in FIG. 5, making it possible to singlepulse the laser system in a rapid fashion and remove the polymerdielectric material 11. The tapered blind via wall 14 (FIGS. 1 and 2) isessential for making the blind via conductive by first electroless ordirect plate and subsequent electrolytic plating methods, especially inmulti-level interconnects.

Three key beam delivery elements are needed to produce a properly debrisfree blind via with a tapered via wall 14:

a) Long Focal Length Lens (17 in FIG. 3)

b) Defocus through the panel (21 and 22 in FIG. 4)

c) Air stream into the window (26 in FIG. 6 and 44 in FIG. 20)

d) For a high-powered system (500 watts or more), off-contact slottedmask 39 in FIGS. 18, 19, 20 and 21 allows a further enlarged beam to beused.

In addition, the laser system needs the following five keyelectronically controlled elements in order to drill cost effectively:

a) RF Controlled Sealed Laser (FIG. 3)

b) Laser Control Box (FIG. 3)

c) Glass Scales (FIG. 3)

d) Motion Positioning Tables (FIG. 3)

e) Computer and Monitor (FIG. 3)

The laser is controlled by electronics in the laser control box whichtakes positional information received from the computer and coordinatesfiring of the laser with positioning of the table. To provide tightpositional and firing control, the table is driven by the control boxand a set of glass scales on the table, with which the board 9 isaligned to a known position, are monitored optically to determineanticipatorily when the laser should be fired. This alignment andcontrol technique allows faster movement with accuracy between drillingpositions than the galvanometer and table combination used by otherlaser drilling systems.

Software that has been used for optimization of mechanical drilling,such as GerbTool, Wise Software Solutions, Inc., Tigard, Oreg., U.S.A.,can be used to optimize laser drilling as shown in FIG. 16 to move themotion control motors over the windows as they are positioned in linewith each other through the Laser Control Box. According to theinvention, by further analyzing the travel path in the optimizationprogram, rotating the table and the travel distance FIG. 17 has beendecreased by as much as 50%. The X/Y tables are then driven in aserpentine fashion allowing the system to drill in both the X and Yaxis. As the software directs the chosen path, the glass scales (FIG. 3)measure both the X and Y positions and feed a signal back through thelaser control box, triggering the laser to fire just prior topositioning over a respective window 12. The laser beam 20 is rapidlypulsed just prior to or just as it enters the window 12 on the panel asshown is FIG. 7. The natural reflection of laser beam 27 from the coppermask 16 on the surface and from the base 16′ of the blind via are madepossible by the low watt density of the defocused laser beam 20 and thewavelength of the sealed RF controlled CO2 laser. The inventiondescribed in the embodied system integration allows the table to move ina continuous motion which is defined as true drill-on-the-fly.

When the key beam elements are properly interconnected to the keyelectronically controlled elements, the invention creates the capabilityof accurately delivering a single pulse laser beam into the respectivewindows 12 to drill vias at a very high speed. The rate at which via canbe drilled is only limited by the maximum rate of the pulsed beam 20leaving the laser aperture 19 and the average speed of the MotionPositioning Table.

The estimated maximum PANEL output for laser blind via drilling with thesystem embodied within this disclosure, using epoxy or polyimidenon-woven aramid dielectric and a laser beam width of 0.762 mm indiameter for a 200 watt RF Excited CO2 sealed laser by Synrad, Inc. TheMaximum Panels per Day is calculated on a 20 hour operating day (3shifts) with 20,000 vias per panels plus a 15 second per panel handlingtime ranges from 1500 to 3000 panels per day with via drilled at 0.32 mmcenters for dielectric thicknesses of 0.2 mm to 0.05 mm.

The estimated maximum PANEL output for laser blind via drilling with thesystem embodied within this disclosure, using epoxy or polyimidenon-woven aramid dielectric and a laser beam width of 0.762 mm indiameter for a 500 watt RF Excited CO2 sealed laser by Synrad, Inc.increases by about 50% for large diameter (0.2 mm) vias. In contrast,production rates for an Nd:YAG laser (266 nm) having a beam diameter of0.1 mm would be more than an order of magnitude loss due to the need totrepan the larger vias. Even for smaller vias, the Nd:YAG must bemultiple-pulse for thicker dielectrics, >0.09 mm, precluding high-speedon-the-fly drilling.

With reference to Table 7, the estimated maximum output in VIAS PERSECOND for laser blind via drilling with the system embodied within thisdisclosure, using epoxy or polyimide non-woven aramid dielectric and alaser beam width of 0.762 mm in diameter for a 500 watt RF Excited CO2sealed laser by Synrad, Inc. The beam diameter may change to accomplishthe appropriate energy range for removing various dielectric materialsalong with lens variations.

Having described and illustrated the principles of the invention in apreferred embodiment thereof, it should be apparent that the inventioncan be modified in arrangement and detail without departing from suchprinciples. I claim all modifications and variations coming within thespirit and scope of the following claims.

What is claimed is:
 1. A laser drilling system for drilling blind viasin printed circuit board panel which includes multiple dielectricpolymer and metal layers, the system comprising: an RF excited CO2 laserof at least 100 watts with the capability to pulse at least 5,000 timesper second to form at least one via per pulse through one or morepolymer layers; an X and Y axis position control system including atable for supporting a panel relative to the laser for drilling and amotion controller; and a controller that outputs a signal to the motioncontroller and receives location input from the controller to actuatethe laser.
 2. A laser drilling system of claim 1, with glass scales inboth X and Y axis that detect position and feed back to the controllerposition information allowing the laser to be pulsed as it crosses orenters the etched windows in both the X and Y axis.
 3. A laser drillingsystem of claim 1, including a computer control system sufficiently fastto provide the optimized drill path data as translated for the motioncontrol system.
 4. A laser drilling system of claim 1, including aslotted off-contact mask that can be rotated a 90 degree angle.
 5. Alaser drilling system of claim 1 in which the table can be rotatedthrough a 90 degree angle to exchange the X and Y axes.
 6. A laserdrilling system of claim 1, including a sufficiently long focal lengthlens to allow a defocused beam to yield a broad watt density having adiameter greater than the blind via to be laser drilled.
 7. A laserdrilling system of claim 1, including means for injecting a flow of airinto the open area of the via mask during laser drilling to assist inthe evacuation of the vaporized dielectric polymer.
 8. The blind vialaser drilling system of claim 1, wherein the radiant energy iscontrolled by pulse duration from the RF excited CO2 laser.
 9. The blindvia laser drilling system of claim 1, wherein the beam is focusedthrough the panel to a depth sufficient to broaden the laser beamdensity at the panel surface, allowing the radiant beam to reflect off acopper mask on the surface of the panel and also reflect off a blind pinat the bottom of the blind via.
 10. The blind via laser drilling systemof claim 1, where focal point of the radiant laser beam is focused atleast 2 mm below the top surface of the panel.
 11. The blind via laserdrilling system of claim 1, including a computer coupled to thecontroller and operative under control of software that has beenoptimized to drill in both X and Y a:s to create a minimal path length.12. A method for laser drilling blind vias at multiple depths in amultilayer panel using as the system of claim 1 and a donut relief on aninner layer of the panel so as to allow a radiant laser beam to reflectoff the surface of the donut to define a part of a side of the blind viaand off a blind pin to define a bottom of the blind via.
 13. A methodaccording to claim 12 in which one of the donut relief and the blind pinare formed by a conductive layer electrically coupled to a circuitformed in the panel.
 14. A method for laser drilling blind viasaccording to claim 12, comprising a single pulse beam delivery to:multiple diameter windows or etched reliefs in the mask, and multipledepth blind pins.
 15. A method for laser drilling blind vias using thesystem of claim 1 by angular alignment of the panel to the X-Y axes ofthe table using pre-etched windows formed in the panel.
 16. A method isaccording to claim 12 for laser drilling a blind interconnect between atleast first and second conductors comprising drilling a blind viabetween the conductors and filling the via with a resistive material tocreate a terminating resistor between the conductors.
 17. A circuitboard is made according to the method of claim 12 havinginterconnections down to one or more of multiple conductive layers fromconductive surface mount pads.
 18. A circuit board according to claim 17in is which the surface mount pads are arranged as a ball grid arrayhaving interconnections down to multiple layers, each multilayerinterconnection being contained within one of the ball grid pads.
 19. Ablind via laser drilling method is using the system of claim 1 fordrilling blind vias in a printed circuit board or multi-chip modulesubstrate, comprising: laser drilling through a pre-defined mask thatdefines the size of the blind vias on circuit boards and polymer basedmultichip modules, using the reflective characteristics of the mask overthe dielectric material and a blind pin under a layer of the dielectricmaterial by IR laser vaporizing dielectric material in the substrate ina single pulse per via.
 20. The blind via laser drilling method of claim19, wherein the laser beam is produced by pulsing a CO2 laser which hasa power of at least 100 watts and the beam is focused through thesubstrate to a depth sufficient to broaden the laser beam density at thesubstrate surface, allowing the radiant beam to reflect off a coppermask on the surface of the panel and also reflect off a blind pin at thebottom of the blind via.